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If there isn't a myth or legend about it, it shouldn't be a "mythical" Pokemon and should just be obtainable normally in-game. Zarude barely passes by virtue of being the protector of a mythical tree, but only just barely.
Deoxys, Diancie, and ESPECIALLY Zeraora (literally nothing special about it) should not be mythical. Magearna is borderline due to age, but is kinda like saying Porygon is mythical-worthy.